If you use polyurethane you want the water based... And this has to gas off completely after your done applying the coats to it. It is strong so I suggest doing it outside or in a garage then let it sit a good week.
You could also try something like silicone I, this is what they use for aquariums. Flex seal also make a liquid paintable product needs to gas off as well...
I am not sure how any of these would work to keep bugs from crawling up it. I have an outdoor enclosure with multiple coats of polyurethane on the wood. And bugs still climb it.
